Starter issue

[FourtyKid]

I have an issue with my starter. When taken out of the car and hooked to a battery with jumper cables, it works great. But when I put it in the car and try to start it with the starter button (switch panel) it doesn't spin. The gear on the starter moves forward when you press the button and moves back when you let go of the button. When the gear moves forward, it never spins. The engine isn't seized up either so I know that's not the issue. If I let the starter hang away from the transmission it still does the same thing. Take it out and hook it to the battery and it spins great. I am completely at a loss as to how to fix this.